Fuel cells have received much recent attention as the next-generation energy conversion technology for portable electronics, stationary power, and automotive applications. There are several types of fuel cells under investigation as classified by their operating temperature and electrolyte, including solid-oxide fuel cells, molten-carbonate fuel cells, phosphoric acid fuel cells, alkaline fuel cells, and polymer electrolyte membrane (PEM) fuel cells. The term proton-exchange membrane is also sometimes used instead of polymer electrolyte membrane, often interchangeably in the case of fuel cells using a hydrogen-based fuel.
PEM fuel cells are the leading candidates for automotive power sources due to their flexible operational conditions, ability to change power output rapidly, and their relative durability and system simplicity compared to the other types. However, there are some critical hurdles for commercializing the current technology. One of the key hurdles is the catalyst in the PEM fuel cell, which impacts on both performance and cost of the fuel cell.
In a typical polymer electrolyte membrane (PEM) fuel cell (FC), the PEM is sandwiched between two electrodes, an anode (negative electrode) and a cathode (positive electrode). The fuel cell includes a supply of fuel such as hydrogen gas to the anode, where the hydrogen is converted to hydrogen ions (protons) and electrons. Oxygen is supplied to the cathode, where the oxygen, hydrogen ions conducted through the PEM, and electrons conducted through an external circuit combine to form water. Catalysts are used to facilitate these electrode reactions. As used in a fuel cell, the catalysts may be more specifically referred to as electrocatalysts.
For better fuel cell performance, the catalyst (such as platinum) is typically in contact with an electron-conducting material such as carbon black (or other graphitic carbon) that conducts the electrons, and a proton conductor (the PEM) that conducts the protons. A typical catalyst is formed from platinum black particles on a carbon support.
The tremendous demand for platinum has greatly increased its cost. Reducing the amount of platinum used in a fuel cell would greatly aid commercialization of this technology. Furthermore, commercialization of PEM fuel cells would be helped by improved catalytic activity and durability of the catalyst under desired fuel cell operation conditions. Most research aimed at stabilizing platinum under fuel cell conditions has centered on alloying platinum with a semi- or non-precious metal such as cobalt. However, other approaches are needed.